Electromagnetic brakes that depend upon friction for their holding power are commonly used with electric motors and other drives. Some of these devices use hydraulic forces to generate high holding torques. Another commonly used device is a magnetic particle clutch/brake. These type of devices use a magnetic field to align magnetic particles in a gap between rotating members and thus create frictional forces which bind the two members together. The magnetic particles can be steel or iron in a powder form and can be suspended in a liquid.
Prior art electromagnetic friction-hold brakes lack sufficient holding force for many applications. Hydraulic friction-hold brakes have the required holding force, but require a hydraulic system with its associated space, weight, complexity, and potential leaks. Magnetic particle brakes are stronger than standard electromagnetic friction-hold brakes, but are limited in holding strength to around 20 psi. In addition, the particles tend to wear out if the brake is allowed to slip, and they require seals to hold the particles in place and do not fail safe (i.e., locked). A permanent magnet system must be added to the magnetic circuit to obtain a fail safe feature.